This description uses the terms defined below.
The term “sampling frequency” describes the clock frequency at which a digital/analog converter (DAC) or an analog/digital converter (ADC) is operated or would need to be operated in order to convert a signal from the digital domain into the analog domain or from the analog domain into the digital domain.
The term “sampling rate” describes the number of samples in a discrete-time signal per unit time, averaged over a defined period.
The term “sampling pattern” describes the periodic structure in which samples of a discrete-time signal are arranged at a defined distance on the basis of a freely selectable reference variable (length, time etc.).
A current example of a mobile radio system is the universal mobile telecommunications system (UMTS). The basic architecture of a UMTS mobile radio system contains, inter alia, mobile stations and subscriber terminals (user equipment (UE)) and a radio access network (RAN). The radio access network contains devices for transmitting data by radio and in addition contains, inter alia, base stations which are named at UMTS Node B. The base stations respectively cover a particular area or a cell in which mobile stations may be located.
In a UMTS mobile radio system, digital data to be transmitted are first of all subjected to channel coding in order to provide the digital data with redundancy. The digital data are then distributed over physical channels by a multiple access method within the framework of the available transmission bandwidth. Finally, the digital data are digitally modulated in order to be transmitted via a mobile radio channel. The mobile radio channel is split in terms of time or frequency by a time-division duplex method (TDD) or a frequency-division duplex method (FDD) for a transmission mode and a reception mode.
The channel coding protects the data to be transmitted against incorrect transmission over a mobile radio channel which is subject to interference, or error correction is made possible on the respective receiver of the data.
In the case of UMTS, the multiple access method used is code-division multiple access CDMA, in which the bipolar data bit stream to be transmitted is multiplied and spread by a subscriber-specific bipolar code sequence or a spreading code. The elements of the spreading code are called chips in order to be able to distinguish them semantically from the bits in the data bit stream. Chips are in principle nothing other than bits. Multiplying the data bit stream by a chip stream produces a bipolar data stream again. Generally, the rate of the chip stream is a multiple of the rate of the data bit stream and is determined by the length of the spreading code, i.e. by a “spreading factor” (SF). The data stream produced by the correct-phase multiplication is therefore at the rate of the chip stream. The multiple access method is used by all subscribers in order to use a subscriber-specific spreading code to provide their user data with a fingerprint which allows the transmitted signal to be restored from the total of the received signals. In the receiver, the bits of the data bit stream can be recovered from the received chip sequence by repeating the process of multiplication. To this end, the chip stream is again multiplied, using the correct phase, by the same spreading code as has already been used in the transmitter, which again results in the transmitted data bit stream. Various data bit streams which are intended to be transmitted in parallel from the transmitter are multiplied by various, orthogonal spreading codes or code sequences and are then added. The total signal is then also subjected to “scrambling”, which is done by multiplying the total signal by a cell-specific or station-specific scrambling code on a chip by chip basis and identifies the cell or the base station.
The time structure of the multiple access is divided into “time frames” of 10 ms each in UMTS mobile radio systems. The duration of a transmission frame or time frame (frame) corresponds to the duration of 38 400 chips (76 800 bits), and the modulation rate is 3.84 Mchips/s. Each time frame is divided into 15 time slots (slots) of length 666 μs, which correspond exactly to the duration of 2560 chips. The chip duration is approximately 0.2604 μs.
The mobile radio channel is characterized by multipath propagation (reflection, diffraction, refraction etc.) of the transmitted signals, time dispersion and Doppler distortion. A radio signal emitted by a transmitter, e.g. a base station, can reach a receiver, e.g. a mobile station, often on a plurality of different propagation paths which differ from one another in terms of different propagation times, phases and intensities. To restore a transmitted signal from a received signal compiled by superimposing signals transmitted on the various propagation paths, said transmitted signal is conditioned using a RAKE receiver in the mobile station or in the base station. The RAKE receiver has fingers which are each associated with a propagation path for a signal and are operated with a sampling delay which compensates for the propagation-time delay of the propagation path in question. Each finger also comprises a correlator which multiplies the time-delayed received signal from a propagation path by a spreading code in order to restore bits from the received signal which the transmitter end has spread using the same spreading code. The output signals from the individual fingers are combined in order to improve the reliability of communication.
In the UMTS mobile radio system, a mobile station needs to synchronize its signal processing apparatuses to received signals which it receives from the surrounding base stations, so as firstly to be able to correctly decode the data which are to be received and so as secondly to be able to generate and transmit transmitted signals, so that said mobile station is in turn understood by the base stations.
When synchronizing a mobile station, individual signal propagation paths or base station signals, which are generally represented by correlation maxima, are identified and normally a base station signal is determined as a reference signal and the mobile station orients the timings and signal processing steps as accurately as possible to this reference signal. In the UMTS standard, an observation interval or observation window is defined around this reference signal and is used to search for further signal propagation paths and also contains all propagation paths which are used for data capture. During synchronization, two problems arise, inter alia, to which the term “synchronization” is linked below.
One problem is that the content of the base station signal or generally of the received signal needs to be analyzed by the mobile station, and characteristic signal contents, e.g. a correlation maximum, needs to be identified. It is therefore necessary to monitor continuously or at particular intervals of time where the characteristic signal contents are located within the received signal and, if their position changes, suitable measures need to be introduced to readjust the mobile station accordingly. If, by way of example, the reference signal moves within the observation window, for example because when a mobile station moves briefly the received signal from this propagation path is compressed in time for this period by the Doppler effect, then the observation window needs to be readjusted to the reference signal so as not to lose the reference signal and to keep it in the center of the observation window.
A further problem is that a timing control device within the mobile station needs to be synchronized to the times at which these characteristic signal contents appear, i.e. to the changed, faster or shifted timing, so that the mobile station can trigger events at defined times relative to the received signal, e.g. can transmit signal contents, in particular. By way of example, the UMTS standard defines for mobile stations that a transmission frame for the transmitted signal starts 1024 chips after the start of a transmission frame in a received signal, specifically with a tolerance of just 1.5 chips (corresponding to approximately 0.39 μs). The mobile station's flow control device thus needs to have its timing readjusted to the appearance of the characteristic signal contents in the received signal.
It should be pointed out that a command on a higher protocol layer can prompt a change of reference signal or of reference base station. In this case, a base station other than the original base station becomes the new reference base station. In this context, the mobile station is instructed to place the center of its observation window on another new propagation path or on another new base station.
One property which is important for synchronizing mobile stations in UMTS mobile radio systems is that apart from a few exceptions, e.g. compressed mode, in which transmission breaks are provided in a time frame, only continuous-time signals are received and transmitted. This has the associated difficulty that readjusting a mobile station with respect to the received characteristic signal content and accordingly also readjusting the flow control device cannot be done abruptly, since otherwise signal contents are skipped or gaps are produced, which inevitably results in errors in the signal evaluation. The readjustment of the mobile station with respect to the received signal, both in terms of content and in terms of time, thus likewise needs to be done continuously or at least in such small steps that there is not yet any significant impairment of evaluation of the signal. In contrast, GSM/EDGE (Global System for Mobile communication/Enhanced Data rates for GSM Evolution) systems involve the performance of a “burst transmission”, in which breaks in transmission and reception periodically arise which can be used for simple abrupt readjustment. For the synchronization with the characteristic signal contents and for the time synchronization, the UMTS standard prescribes the following conditions:                The mobile station needs to be readjusted up to a stipulated maximum permissible error, which defines the resolution of the control. In addition, there is an upper limit for a control error.        The control error must be compensated for within a particular time, e.g. the duration of a transmission frame (10 ms). This stipulates a minimum speed for the readjustment. The maximum timing error for a mobile station is determined by the delay time between the appearance of a control error and the correction of a control error.        When readjusting a transmitted signal transmission time stipulated by the flow control device, which is in turn readjusted to the received signal, it is not possible to exceed a maximum permitted change speed, e.g. ¼ chip in 200 ms. For this reason, the change speed used to readjust the timing control device is likewise subject to this stipulation.        In line with the UMTS standard, characteristic signal contents should be identified with at least an accuracy of half a chip period, and the same accuracy should be used to readjust the mobile station.        In line with the UMTS standard, the UMTS mobile station's flow control device should be able to be set at least with an accuracy of half a chip period and should be able to be readjusted to the characteristic signal content of the received signal with the same accuracy.        
A further constraint when synchronizing mobile stations in the UMTS radio telecommunication system is that the mobile station's available operating clock for the signal processing devices and for the flow control device is frequently not at a frequency which is an integer multiple of the UMTS typical chip frequency of 3.84 MHz, but rather is derived from a GSM-typical clock (e.g. 13 or 26 MHz), for example. It is thus not possible to map UMTS events onto the operating clock with precision timing.
Known arrangements and methods for synchronizing mobile stations in the UMTS radio telecommunication system operate on the basis of the principle of inserting individual samples into the received or transmitted signal or removing individual samples from the received or transmitted signal for the purpose of readjustment and synchronization.
One drawback of these arrangements and methods is that they require the signal normally to be at or to be processed at a sampling frequency which is several times higher in comparison with the sampling theorem in order to obtain sufficiently fine resolution. This normally requires increased circuit, memory and/or computation complexity.
A further drawback of these arrangements and methods is that the number of samples per transmission frame or time slot is not constant, which results in problems with multiplying the signal by scrambling and spreading codes, for example, and therefore requires special consideration.